Here the Mishnah is going to talk about what happens with that became impure. Could we redeem them or not? The Mishnah says: If you have a minchah (meal offering) or some of the nesachim (libation offerings) that became tameh (impure)—if a person dedicated flour for a minchah or wine for the nesachim, and they became impure before being placed into the kli sharet (Temple service vessel) and becoming fully sanctified—they still have the ability to be redeemed, because items that are only verbally designated for the menachot (meal offerings) or the nesachim and have not yet been placed in the kli sharet only have kedushat damim. They only have a monetary sanctification. Kedushat damim. So, therefore, it still has the ability to be redeemed if it becomes impure.
So it's no longer fit to be offered on the Mizbeach (altar). Therefore, a person could redeem it. The money that he redeems it with will become sanctified in its place and has to be used to buy flour or wine for minchah or nesachim. And the flour and wine that he had become chullin, a regular mundane object that he could use on his own. Because, again, we're talking here about kedushat damim, monetary sanctity that could be redeemed if it becomes impure.
Mishekidshu bakli ein lahen pidyon (redemption). But if it becomes tameh after it was sanctified in the kli sharet, they cannot be redeemed, because once the flour or the wine was placed into the kli sharet, it has kedushat haguf. It has a physical holiness to it. And at that point in time, there's no idea of redemption. There's no idea of pidyon.
Ha'ofot (birds) that were designated for a korban (sacrifice), ve'ha'etzim (wood) that was dedicated as fuel for the Mizbeach, ve'ha'levonah (frankincense) that was consecrated to also be offered on the Mizbeach, and u'klei sharet, or the kli sharet themselves that were already consecrated, ein lahen pidyon, cannot be redeemed, even if they become tameh, since they automatically possess kedushat haguf.
Shelo ne'emar ela behemah. In other words, an animal offering that does become blemished could be redeemed. We say that's the exception to the rule. Whenever we say that there's an idea of pidyon, of something that has kedushat haguf, it's only talking about an animal that develops a blemish. But everything else that has kedushat haguf can never be redeemed.